The safety and performance of gun barrels can be compromised by use, wear, erosion, and other defects that may develop during or after manufacture, due to rough handling, under the forces related to firing, as well as under the environmental conditions and handling in the field.
Currently, the typical practice to inspect gun barrels is performed manually. However, this type of inspection is subjective and prone to operator error. Automated systems that scan the inside diameter of the gun barrels, in both manufacture and in the field, are complex, bulky, expensive, and relatively slow.
Various dynamic methods are known for identifying and quantifying structural damage as a change in fundamental resonant frequencies, which occurs as a result of such a defect in a solid structure. The change in frequencies can often be used to detect and locate the defect, even in the presence of ambient noise. A significant amount of work in the field relates to one-dimensional problems, dealing with cracked beams under axial and transverse vibration due to the ease of modeling a real beam or rod and thereby simplifying the analysis.
Particular examples of such work including a simple theory of cracked beam under axial and transverse vibrations, is described in Y. Narkis, “Identification of Crack Location in Vibrating Simply Supported Beams” Journal of Sound and Vibration 172(4), 549-558 (1994). Another work in P. F. Rizos et al., “Identification of Crack Location and Magnitude in a Cantilever Beam from the Vibration Modes”, Journal of Sound and Vibration 138, 381-388, 475-488 (1990) and A. D. Dimarogonas, “Vibration of Cracked Structures: A State of the Art Review”, Engineering Fracture Mechanics 55(5), 831-857 (1996), also disclose such methods and review the field of crack detection using frequency spectra.
Mathematical models are developed that simulate a crack as a linear spring for axial motion and as a torsion spring under transverse motion. The compliance of the springs is represented by the stress intensity factor based upon disclosures by G. R. Irwin, et al., “Fundamental Aspect of Crack Growth and Fracture, Fracture, and An Advanced Treatise,” Vol. III. Engineering Fundamentals and Environmental Effects, edited by H. Liebowitz, Academic Press, New York, 1971, pp. 2-46. In this publication, it is shown that the natural frequencies of cracked rods and beams shift to lower values under axial or transverse loads because of the increased compliance.
A particular dynamic method was disclosed by A. Morassi, in a paper titled: “Identification of a Crack in a Rod Based On Changes in a Pair of Natural Frequencies,” Journal of Sound and Vibration, 242(4), 577-596 (2001), wherein a series of calculations and experiments were presented with a hypothesis expecting more reliable results, when the damage being identified was less severe and lower order frequencies were considered. Morassi concluded that his analytical model, with these factors of less damage and lower frequencies, proved extremely accurate—the percentage discrepancy between the measured and analytical values of the involved natural frequencies being lower than 1% within the 30th vibrating mode. Morassi's method included a series of experiments using an impulse force hammer to excite a steel rod of square solid cross-section to detect notches of increasing depth (damage)—the rod suspended by two steel wire ropes to simulate free-free boundary conditions, with the axial response measured by a piezoelectric accelerometer fixed in the center of an end cross-section of the rod. The vibration signals were acquired by a dynamic analyzer and then determined in the frequency domain to measure the relevant frequency response term (inertance)—using methodology detailed in a 1997 article by A. Morassi, in Inverse Problems in Engineering, 4, 231-254, titled “A Uniqueness Result on Crack Location in Vibrating Rods”.
An alternative dynamic method using impact-acoustic resonance, including Impulse Resonance Acoustic Spectroscopy (IRAS), was detailed by A. Sutin, in a presentation at the 35th Annual Review of Progress in Quantitative Nondestructive Evaluation, Chicago, Ill., 2008—the presentation titled: “Application of Impulse Resonant Acoustic Spectroscopy (IRAS) for Crack Detection in Pipes”.
In IRAS, a laser vibrometer is used to detect the vibration of the specimen's surface. The spectra of the received laser signal is analyzed using FFT, to transform the signal to the frequency spectrum, such that the narrow frequency band about the specimens' resonance frequency can be filtered and isolated, and the envelope function of that filtered signal established—which will indicate a clear splitting of the resonance frequency envelope in the presence of a crack. This methodology has been demonstrated on thin-walled solid geometries, such as casing pipes, and involves significant expense.
In addition, the safety and performance of the gun systems can be compromised when the safe service life has been reached or if there is significant wear and erosion, as described earlier. The gun tube is condemned after a predetermined number of rounds have been fired or on the basis of a visual inspection.
There is currently no system that quickly and automatically determines if the wear and erosion of a fielded tube is excessive or if the gun is approaching its safe service life. The gun tubes are inspected manually (visually) at regular intervals to identify defects and changes in geometry and to determine if the wear, erosion, and defects are sufficient to warrant the removal of the tube from service. These manual visual inspection procedures using, for example borescopes, magnetic particle inspection, and pullover gages might be subjective and inefficient.
Certain military procedures, such the Weapon Record Data Cards (DA 2408-4) are used to determine if the safe service life of the tube has been reached. These cards must be accurately maintained by soldiers throughout the life of each tube and stored with the tube. If the cards are missing or incomplete, the tube must be immediately inspected or condemned, possibly prematurely. The procedures for assessing the health of a fielded gun contribute to the high operation and maintenance (O&M) costs. These O&M costs are a significant part of the military budget and are expected to increase as vehicle fleets age and as new lightweight systems are deployed.
Another conventional method for determining if the gun system is approaching the end of its safe service life, is to use an automatic round counter for some small arms. This method has also been proposed for use with other gun systems. However, the automated round counter adds additional weight, cost, size, and complexity to the gun systems. It also increases the logistic burden if it requires an external power source, such as a battery, to operate. The operational requirements of a gun system may result in an extreme thermal or shock environment that precludes the use of automated counters.
The following are exemplary round counters: The Weapon Shot Counter, available from Accu-Counter Technologies, Inc. PO Box 18038, Erlanger, Ky. 41018-0038; and the Weapon Shot Counter available from Advanced Design Consulting USA, Inc., 126 Ridge Rd, PO Box 187, Lansing N.Y. 14882. Another exemplary round counter design is described in U.S. Pat. No. 7,716,863 to Johnson et al.
Yet another conventional method to determine if the wear and erosion of a gun system is excessive or if the gun is out of tolerance is to use automated systems to detect localized defects with transducers that move across the tube surface, or that are stationary relative to a moving tube. However, these automated systems may prove to be costly, slow, and cumbersome.
An exemplary system for detecting localized defects with transducers, is the ROBINICA Robot Inspection and Calibration System that is available from Dacon A S (Postbox 133, Gamle Ringeriksvei, 1321 Stabekk, Norway). Another system is the Field Inspection Vehicle, which is generally described at: (http://www.amsnt.com/micro_electronics_field_inspection_vehicle.html), and which is developed by American Science and Technology, Benet Laboratories, and South Dakota State University. These two exemplary systems perform inspections using a tethered measurement unit inserted into the barrel. The measurement unit collects data as it traverses the bore and may also provide an output from a camera that is integral to the device, as the measurement unit performs the scanning operation.
Still another conventional method proposes the use of guided microwave signals and acoustic techniques to identify gross defects in pipes and tubes without the need for moving a transducer along the surface. However, these guided microwave signals have been shown to detect large cracks using differences in the magnitude of the reflection coefficient. This approach may lack the sensitivity to identify subtle changes in tube geometry or changes in the bore surface due to firings.
In addition, the current automated acoustic techniques that do not employ a moving transducer relative to the inside or outside of a tube surface may lack the sensitivity to detect small defects in the bore surface or changes in the bore surface properties due to firings.
In the International Journal of Applied Electromagnetics and Mechanics, Volume 20, numbers 3-4 (2004) pages 171-178, Shibata, et al. disclose a method of using electromagnetic waves for detecting cracks in pipes in a paper titled “Crack detection method using electromagnetic waves.” The paper describes an approach based on the difference in the intensity microwaves for tubes with and without a crack. Tests were performed on a 34 mm diameter pipe using 2 mm thick spacers with 38 mm and 42 mm diameters to represent cracks. The crack depth was shown to correlate with the change in magnitude of the transmitted wave. A second experiment was performed showing the effect of crack position on the intensity of the reflected wave.
The Journal of Materials Processing Technology 161 (2005), pages 348-352, Shibata, et. al. discusses a similar approach in a paper titled “Experimental study on NDT method using electromagnetic waves”. Abbasi, et. al disclose a microwave inspection technique in the Journal of Power and Energy Systems, vol 2 No. 2 (2008) pages 538-544 in a paper titled “Microwave Detection of Longitudinal Crack and Identification of its Location in a Straight Pipe” and in the international Journal of Pressure Vessels and Piping 86 (2009) 764-768 in a paper titled “Detection of axial crack in the bend region of a pipe by high frequency electromagnetic waves”. The approach is based on the differences in the reflection coefficient (ratio of reflected to transmitted signal) between a defect-free structure and one with a crack. Crack location is determined by measuring the time-of-flight of the electromagnetic waves using the inverse fast Fourier transform of network analyzer signals.
Thus, there still remains a need for a relatively low cost and simple system and associated method for expeditiously determining if the safety and performance of a gun tube have been compromised by firing damage or by an excessive number of fatigue cycles. Prior to the advent of the present invention, the need for such a system has heretofore remained unsatisfied.